The invention belongs generally to the field of flame-proof walls, barriers and fittings for use on or with explosion-proof enclosures. The invention can be applied to any large wall or cover through which hot, exploding gases can move with very little back pressure but which will effectively block the passage of flame. In particular, it is concerned with a flame-proof wall or cover to make electrical equipment safe in potentially explosive atmospheres such as in underground coal mines. It is especially useful on very large structures such as heavy duty transformers which could advantageously be located near a mine face.
Underground coal mining machinery is powered by many different kinds and sizes of direct and alternating current electrical motors. To supply the power conversion and control equipment for these motors, mines must have a wide variety of electrical apparatus including transformers, power centers, rectifiers, battery chargers, motor controllers, contactors and switches. These generate heat, sparks and arcing and can ignite and explode methane-air mixtures which are often present. Accordingly, all such underground electrical equipment, installed in bye of the last open cross cut, must, by law, be housed in explosion-proof enclosures.
Theoretical considerations indicate that the temperature rise produced by the explosion of a stoichiometric mixture of methane and air contained at atmospheric pressure in a sealed enclosure will result in a pressure increase of approximately 150 pounds per square inch. Further, the disposition of equipment within the enclosure can bring about dynamic effects during the course of an explosion (known as "pressure piling") which result in peak explosion pressures significantly greater than 150 p.s.i.
It will thus be evident that the enclosures for explosion-proof electrical equipment have to be of much heavier construction than those commonly used for general industrial applications. Moreover, because pressure is force per unit area the total force on, for example, the cover of an explosion-proof enclosure, is related to the square of its linear dimensions. Thus the design and manufacture of large explosion-proof enclosures becomes increasingly difficult and expensive.
Virtually all electrical components produce heat, which is in most cases undesirable. The dissipation of this heat to prevent damage to the components is important in the design of electrical equipment. The most common form of cooling is natural convection ventilation by ambient air, through louvers or similar openings in the sides of enclosures. Such simple means of cooling are not applicable to explosion-proof enclosures because in the event of an explosion within the enclosure, the flame would readily be propagated to the surrounding atmosphere which might also be an explosive mixture.
It is possible to design special ventilators or fittings for explosion-proof enclosures which permit some ventilation, or breathing, but in the event of an explosion within the enclosure would prevent flame from being transmitted to the outside. Such devices, sometimes known as flame arresters or breathers, have been known for over 160 years, the basic principle being that employed in the original miners' safety lamp credited to Humphrey Davy where a cylindrical brass screen surrounding a flame cooled the escaping combustion gases down to a temperature insufficient to ignite explosive gases outside the screen.
The provision of an adequate ventilator can substantially reduce the maximum explosion pressure to which an enclosure is subjected and thus the enclosure can be constructed without the need for massive steel sections capable of withstanding high explosion forces and with consequent savings in labor and material costs. Furthermore, the reduced weight of such an enclosure is a continuing advantage where the electrical equipment must be moved from time to time as the mining work progresses.
The great majority of explosion-proof enclosures in current use have no means of direct ventilation. Heat dissipation from heat-generating circuit components within the enclosure is by internal air convection which transmits the heat to the cooler walls of the enclosure. The metallic walls conduct the heat through to their outside surfaces where cooling by external ambient air convection takes place. Hence, it will be apparent that in the case of explosion-proof enclosures there are two additional gas-to-solid and solid-to-gas heat transfer processes involved in the heat dissipation path, each of which requires a substantial temperature gradient to be effective.
This indirect transfer of heat from the inside to the outside of explosion-proof enclosures requires substantially higher driving temperatures than in freely ventilated enclosures. It is well established that the service life of most commonly used electrical insulating materials is inversely proportional to the operating temperature, and it is therefore desirable to reduce the temperature whenever expedient.
Attempts to ventilate the interior space of explosion-proof enclosures by passing relatively cooler ambient air through them have been effective on some electrical motors where it has been possible to modify the rotor to act as a fan to draw outside air in through one flame arrester and discharge it through another. This is illustrated for example in U.S. Pat. No. 2,789,238 issued to J. H. Staak. However, in explosion-proof enclosures such as those for transformers, power centers, rectifiers, battery chargers, and the like, which do not already have internal motors capable of being used as fans, or in which the additional complication of a separate motorized fan would be objectionable, the heat build-up resulting from the lack of ventilation has been tolerated as the least objectionable of undesirable choices.
Prior to the present invention, large area walls capable of providing ventilation and being effective as a flame-proof barrier have not been available. As a result, large enclosures such as power transformers used in underground coal mines have not been made explosion-proof. For this reason, they have been located a long distance from the potentially gassy coal face area where most of the electrical power is used to mine the coal. This results in increased voltage drop, reduced motor starting torques, and lower efficiencies. It also requires a long length of large diameter, low-voltage cable, which is costly and hard to handle, between the transformer and the machinery at the mine face.
Flame arresting ventilators and breather pipe fittings of relatively small sizes have been used widely in underground electric motors, venting and drain devices for petroleum and explosive chemical tanks, for exhausts of engines operating in potentially explosive atmospheres, and many other applications involving explosive gases. These are in four general types as follows:
I. Closely-spaced flat or coiled metal strips or plates, which allow the free passage of gases, but in case the gases are ignited, they will be cooled below the ignition temperature by the time they pass through to the other side. Examples are shown in the following U.S. patents: Long et al U.S. Pat. No. 2,068,421; Anschicks U.S. Pat. No. 2,151,180; Edwards U.S. Pat. No. 2,247,225; Duggan U.S. Pat. No. 2,388,395; Lisciani U.S. Pat. No. 2,691,464; Staak U.S. Pat. No. 2,789,238; Norton U.S. Pat. No. 3,903,646; and Szego U.S. Pat. No. 4,149,649.
II. Heat conducting open porous metal flame-proof barriers. Examples are shown in the following U.S. patents: Immel U.S. Pat. No. 2,801,768; Kleinpeter U.S. Pat. No. 2,985,337; Grady, Jr. et al U.S. Pat. No. 3,394,843; Gurney U.S. Pat. No. 3,711,259; and Gunderman et al U.S. Pat. No. 4,180,177.
III. Flame-proof cylindrical screens similar to that used on the original Davy-type mine lamp. Examples are shown in the following U.S. patents: Jett U.S. Pat. No. 2,220,720; and Dingman U.S. Pat. No. 2,515,950.
IV. Apertured or truncated disks or plates which are clamped together around their outer peripheries and supported at right angles to the general direction of fluid flow. The successive disks or plates have offset openings forcing gases to follow a tortuous path from one to the next. This type is commonly in use, but its air flow capacity is limited because the disks or plates are completely unsupported inside their clamped outer edges. If made in large sizes as contemplated by the present invention, they would distort and allow the passage of flame to the outside in case of an internal explosion. Examples are shown in the following U.S. patents: Wyman U.S. Pat. No. 1,960,259; and Spaeth U.S. Pat. No. 2,610,684.
All the conventional ventilating flame-proof devices described above are relatively small, in the nature of pipe fittings or the like. None discloses a large wall section which is capable of passing substantial volumes of air or gases at low back pressure while acting as a flame arresting barrier to fill the need for large explosion-proof enclosures such as for power transformers used in mines.